Painting operations serve as a major production bottleneck in aircraft manufacturing and there is great interest in the industry to improve efficiency. On average, it takes 5 to 7 days to paint an entire aircraft during production. Coatings are usually manually sprayed instead of robotically sprayed because of the shape and size of the aircraft. In addition, to achieve the best performance and appearance and to avoid sagging from over-spray, one layer of coating is usually applied by two to three passes of the same coating in a period of 10 to 60 minutes. The second pass of coating must be able to wet well with the first pass of coating to avoid film defects such as orange peel, low gloss and high haze.
Polyurethane coatings are usually employed for aircraft because they have excellent chemical resistance, durability, low temperature flexibility, and heat stability. However, they generally cure slowly and have long dry-to-tape time (up to 20 hours). The slower the dry-to-tape time, the longer it takes to complete the painting cycle. Developing faster drying paints is an attractive approach to improving efficiency.
The addition of catalysts to the polyurethane coating can accelerate the drying process and shorten the dry-to-tape time. However, viscosity of the coating composition may increase too quickly for consistent spray application. Performance of the coating at the end of its pot life may also be different from that of the freshly mixed paint and sometimes coatings cannot meet specification requirements such as adhesion, chemical resistance and appearance. If the pot life is too short, the performance and appearance of the coating on one area of the aircraft could be unacceptably different from another area. Also, in the case of accelerated curing, the reaction is very fast and rapidly converts low molecular weight resins to a high molecular weight polymer with high crosslink density. This makes it very difficult for the next coating layer to blend with a previously applied layer, particularly at seams or edges, and often results in poor appearance such as high haze and low gloss in the overlap areas. This is commonly referred to as poor wet-edge. Wet-edge time is the interval of time in which a fresh layer of paint can blend into a previously applied paint when only part of the surface is sprayed with a fresh paint. Wet-edge time is especially important for painting an aircraft because multiple work crews are involved in the operation: the areas where paint layers applied by different teams overlap will look striped and hazy if the wet-edge time of a coating is too short. Short wet-edge time will prevent overspray from blending into a previously applied paint film, resulting in a rough surface. Streaks, tiger stripes, or other visual irregularities are not acceptable on commercial airplanes and a wet-edge time of at least 30 min is generally required for all aerospace topcoats.
It is desirable to provide a polyurethane coating system that has good pot life and long wet-edge time, but that cures fast with a short dry-to-tape time.